High molecular weight epoxy resins have been prepared by reacting low molecular weight epoxy resins with dihydric phenols in the presence of phosphonium catalysts such as disclosed by Dante and Parry in U.S. Pat. No. 3,477,990 and by Perry in U.S. Pat. No. 3,948,855. While the phosphonium halide and carboxylate complex catalysts disclosed therein do indeed prepare high molecular weight epoxy resins, it would be desirable to be able to prepare high molecular weight resins with certain improved properties in the process employed to produce them and/or in the products so produced.
In the production of high molecular weight resins, it is often desirable to employ temperatures in excess of 200.degree. C. so that the viscous products can be physically agitated or transferred. However, at these temperatures, the conventional phosphonium catalysts are deactivated prematurely, resulting in incomplete reaction and products containing high levels of unreacted phenolic hydroxyl groups and low equivalent weight or low molecular weight products are produced.
It would be desirable to employ a process using a catalyst which is more active at elevated temperatures so that for the same ratio of reactants employed, higher equivalent weight or molecular weight epoxy resins can be produced which contain lower amounts of residual or unreacted phenolic hydroxyl groups.
Also, one convenient method for packaging epoxy resins which are to be advanced into higher molecular weight epoxy resins is to mix the low molecular weight epoxy resin with the catalyst; however, with the conventional phosphonium catalysts, upon prolonged storage at elevated temperatures, the catalyst tends to become deactivated or at a minimum loose its activity. It would also be desirable to employ a catalyst in these precatalyzed packages or mixtures which does not become deactivated during storage at elevated temperatures or at a minimum does not loose as much of its activity at elevated temperatures.